Add wrappers for `SO_PEERCRED` and `SCM_CREDENTIALS` #3378

Job	Run time
Check --no-default-features (nightly)	1m 15s
Check nightly-only targets (nightly)	36s
Check selected Tier 3 platforms (nightly)	4m 33s
Check (stable)	8m 13s
Rustfmt	7s
Check (nightly)	9m 58s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 31s
Check (1.63)	19m 2s
Test (ubuntu)	1m 14s
Test use-libc (ubuntu)	1m 27s
Test (ubuntu-20.04)	1m 2s
Test use-libc (i686-linux)	1m 24s
Test (i686-linux)	1m 38s
Test use-libc (aarch64-linux)	2m 11s
Test (aarch64-linux)	1m 40s
Test use-libc (powerpc64le-linux)	1m 21s
Test (powerpc64le-linux)	1m 44s
Test use-libc (riscv64-linux)	1m 26s
Test (riscv64-linux)	1m 50s
Test use-libc (arm-linux)	1m 44s
Test (s390x-linux)	2m 6s
Test (arm-linux)	1m 48s
Test (ubuntu-stable)	1m 14s
Test (i686-linux-stable)	1m 46s
Test (aarch64-linux-stable)	1m 27s
Test (riscv64-linux-stable)	1m 21s
Test (s390x-linux-stable)	2m 16s
Test (powerpc64le-linux-stable)	2m 3s
Test (arm-linux-stable)	1m 21s
Test (ubuntu-1.63)	4m 44s
Test (i686-linux-1.63)	5m 8s
Test (aarch64-linux-1.63)	6m 28s
Test (riscv64-linux-1.63)	5m 17s
Test (s390x-linux-1.63)	5m 7s
Test (powerpc64le-linux-1.63)	4m 58s
Test (arm-linux-1.63)	6m 41s
Test (macos-latest)	1m 14s
Test (macos-11)	1m 9s
Test (windows)	1m 23s
Test (windows-2019)	1m 14s
	2h 2m 41s

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